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Moon and Antares

StarDate:

January 10, 2010

The Moon bears down on a giant star before dawn tomorrow: Antares, the heart of Scorpius. From most of the country, they'll be separated by a fraction of a degree -- less than the width of the Moon itself.

In fact, for some parts of the northeastern U.S., the Moon will actually cover up the star for a little while. Unfortunately, that'll take place after sunrise. You can see it through binoculars or a telescope -- just be careful not to aim toward the Sun!

Such an event is called an occultation, from a Latin word that means "to hide."

Antares is one of a handful of bright stars that the Moon can cover up. These stars lie quite close to the ecliptic -- the Sun's path across the sky. The Moon's path is tilted a little with respect to the ecliptic, so it doesn't cover these stars every time it passes by them. But it can cover them up when its path crosses the ecliptic -- which is what's happening tomorrow.

Astronomers have used occultations to measure the sizes of stars -- and Antares is a good example. They found that it's more than 600 million miles in diameter -- a measurement confirmed with other techniques. To put that size into perspective, if the Sun were the size of a baseball, Antares would be bigger than a baseball stadium's infield. And the Moon would be smaller than a grain of sand.

Look for Antares quite near the Moon early tomorrow. They rise in the southeast a couple of hours before sunrise.

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